Highlight config documentation version

[Herrtian]

Closes #4664

Keeps the search/version controls visible while scrolling and adds a short note that config options vary by rustfmt version.

Checks:

• git diff --check

[ytmimi]

@Herrtian Could you post a screenshot of what the docs site looked like before and after this change? Would be good so that we can visually check to see that the updated CSS makes the version stand out more.

If you've got python installed, you can run a dev server from the root of your local rustfmt checkout with

python -m http.server 8000 -d docs/ 

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[Herrtian]

Here are the before and after screenshots.

Before:

After:

@rustbot ready